There has been a long felt desire of being able to introduce a liquid fabric softener directly into an aqueous solution of a laundry detergent. The advantages of such a mixture are obvious, the primary one being the simplification of the laundering task. A very difficult problem, however, is that the most effective detergents are anionic surfactants, the most effective fabric softeners are cationic surfactants and those two types of surfactants are not compatible in a common mixture.
Such incompatibility is due to both the cationic and anionic ingredients losing their respective surface active properties due to the interaction between the negatively charged hydrophilic group of the anionic surfactant and the positively charged hydrophilic group of the cationic surfactant. Such interaction generally results in the loss of the desirable surface active properties of each of the two types of surfactants. There are many examples in the art, however, with varying degrees of success, of attempts to form effective mixtures of anionic and cationic surfactants by the use of additional ingredients intended to minimize such loss of desirable properties.
U.S. Pat. No. 3,668,136 to Parker discloses an anionic and cationic surfactant mixture which was said to be compatible, without the need of ingredients, such as propylene glycols, by selection of a highly complex cation comprising a quaternary ammonium compound containing substituted groups such as alkoxy and piperidine groups. The aqueous solution shown in Parker having the highest concentration of Parker's composition is that of the shampoo of Example 5 which contains 15% of anionic surfactant, 8% of cationic surfactant and 73% water.
U.S. Pat. No. 3,703,480 to Grand et al discloses cationic softeners and anionic detergent mixtures with aminopolyureylene resin in dry particulate form until used in a highly diluted aqueous solution.
U.S. Pat. No. 3,997,453 to Wixon discloses cationic and anionic surfactant mixtures which may contain up to 5% anionic surfactant at least part of which may be sodium xylene sulfonate, and which is in the form of an opaque liquid.
U.S. Pat. No. 4,230,590 to Wixon discloses cationic softener and anionic detergent mixed with a fatty acid soap cellulose ether mixture to provide a dry shaped particle.
U.S. Pat. No. 4,255,294 to Rudy et al discloses a dry powdered mixture of a cationic nitrogenous compound and an anionic organic detergent as well as other ingredients, including sodium xylene sulfonate.
U.S. Pat. No. 4,272,395 to Wright discloses a cationic and anionic surfactant mixture including a short chain anionic surfactant which may comprise sodium xylene sulfonate and which may be in the form of a translucent liquid having up to 20% active detergent content.
U.S. Pat. No. 4,302,364 discloses a liquid detergent containing anionic and cationic surfactants and a nonionic ethoxylated component. The detergent may also contain a solvent such as one selected from the group consisting of lower aliphatic alcohols having from 1 to 6 carbon atoms and from 1 to 3 hydroxyl groups. The concentration of the cationic component in the detergent may be as high as 15%. The anionic surfactant may be an alkyl benzene sulfonate, but the alkyl group must contain from about 8 to about 15 carbon atoms.
U.S. Pat. No. 4,326,971 to Wixon discloses dry blended particles of a detergent composition comprising cationic, anionic and nonionic surfactants.
U.S. Pat. No. 4,507,219 to Hughes discloses liquid detergent compositions containing up to 15% anionic sulfonate which may be alkali metal salts of alkylbenzene sulfonates in which the alkyl group contains from about 10 to about 15 carbon atoms. Up to 5% of a cationic cosurfactant may also be present. Propylene glycol may be used with the detergent as part of a solvent system.
Japanese publication No. JP81055497 teaches that a cationic and anionic surfactant liquid detergent composition may be stable in the absence of additives such as propylene glycol and sodium benzene sulfonate and sodium p-toluene sulfonate. The Example shows 20 wt.% total of anionic surfactant and 2.7 wt. % of cationic surfactant. Over 70 wt. % of the detergent is water.
The present invention is based on the surprising discovery that it is possible to obtain a stable clear liquid solution which contains both anionic and cationic surfactants by adding to the solution an effective amount of propylene glycol and an alkali metal salt of benzene sulfonate, toluene sulfonate or xylene sulfonate. The mixture may then be employed as a fabric softener by adding it directly to an aqueous detergent solution. The detergent solution will thereby acquire a softening function without sacrifice of detergent strength.